The present invention relates to an intake air amount control apparatus of an internal combustion engine which is mounted in, for example, a vehicle, and mainly concerns a control means for feeding back the engine speed to calculate a target engine speed. More particularly, the present invention relates to an improvement in an open-loop control mode of such a control means when the feedback loop control mode is switched to the open-loop control mode.
In internal combustion engines, in order to warm up the engines and to stabilize an idling speed, the idling speed is set to be high while the temperature of cooling water thereof is low. For this reason, when the temperature of the cooling water is low, that is, when warming-up of the engine has not been performed, the control amount of intake air to the engine must be large.
In such an engine idling state, the target engine speed corresponding to, for example, the temperature of cooling water is set and an actual engine speed is fed back to set this actual engine speed at the target engine speed. Techniques shown in U.S. Pat. No. 4,237,833, U.S. Pat. No. 4,306,527 and U.S. Pat. No. 4,344,399 are known as such an engine speed control means.
Except for the engine idling state in which the feedback-loop control mode is set, the open-loop control mode is set. When the valve opening position for controlling the amount of intake air in the open-loop control mode is set at a predetermined position, and when this open-loop control mode is switched to the feedback control mode, the valve opening cannot maintain the target engine speed in the corresponding feedback loop control mode. Therefore, in this feedback control mode, the actual engine speed becomes extremely high or extremely low.
In order to solve such a problem, the control amount "Dthw" of the intake air in the open-loop control mode is preset with respect to the engine speed, as shown in FIG. 1. Then, in the feedback loop control mode, the valve opening is controlled in accordance with this preset control amount value, as shown in FIG. 2.
However, in practice, the control device of an intake air amount of the actual internal combustion engine has flow characteristics that are different than the predetermined values which depend upon variations in the leak-flow amount when a throttle valve is fully closed and depend upon friction between the components thereof, resulting in a deterioration over time of its characteristics. Therefore, with respect to a plurality of engines or to a single engine, the control amount deviates from a predetermined value over time. For this reason, in the extreme case, as shown in FIG. 3, the control amount in the open-loop control mode becomes smaller than that corresponding to the target engine, thereby causing engine trouble such as stalling, especially when the vehicle's speed is decreased.